The proposed experiments are designed to use two approaches, acute neurophysiological techniques and psychophysical methods, to study aspects of the central systems involved in pain perception and its regulation. The neurophysiological studies will investigate the action of four descending systems, the raphe-spinal system, the reticulospinal system, and descending projection from the sensorimotor cortex and the orbital cortex, on the cells of origin of three ascending projections, the spinocervical trait and the dorsal spinocerebellar trait. Emphasis will be placed on using conventional histogram techniques, cross correlation analysis, and Wiener kernel analysis to determine how the descending systems modify the responses of these neurons to noxious and nonnoxious inputs applied to the center and the periphery of the receptive fields. Another series of experiments will use pharmacological blocking agents to investigate the role of serotonergic and opiate systems in mediating the inhibitory effects resulting from stimulation of the nucleus raphe magnus. The psychophysical experiments will examine the role of distraction and motor activity on the perception of noxious and nonnoxious thermal stimuli using a paradigm based on signal detection theory. Other psychophysical studies will evaluate the interaction of transcutaneous electrical stimulation and aspirin. Additional experiments will study the changes in the perception of acute thermal stimuli resulting from any long-term modifications in sensory function assocated with the chronic pain state. A final group of psychophysical studies has been designed to examine the perceptual changes induced by implantable electrical stimulating devices used to treat this condition.